This invention relates to forming a tungsten oxide film on a substrate by applying an alkyl amine tungstate compound thereon and pyrolyzing at least a portion of the alkyl amine tungstate compound to form the tungsten oxide film. More particularly this invention relates to such method utilizing a solution containing the alkyl amine tungstate compound to uniformly apply the compound onto the substrate, drying the solution to form a deposit and heating the deposit for a time and at a temperature sufficient to achieve a desired composition of the tungsten oxide film.
In addition, the method includes forming a color gradient. That is, regions of the film having different colors are made by varying the time and temperature of heating for respective regions of the film. The regions are faint yellow to deep brown, and electrochromic. The method also results in the formation of suboxides of tungsten oxide (WO.sub.3) at selected times and temperatures.
Tungsten is among the transition metals which form electrochromic metal oxide films. Electrochromic materials have variable light transmittance in response to an applied electrochemical potential. Such metal oxide films are used in electrochromic display devices wherein the film changes color when subjected to an electrical potential. A tungsten oxide film is typically preferred for this application due to its highly visible color change.
Tungsten oxide films are of interest for use as a coating for glass, to produce windows with controllable light transmission. For example, the automobile industry might use such coated windows to lower the amount of sunlight-generated heat in the passenger compartment of a car. The tungsten oxide, WO.sub.3, film is normally a faint yellow, and when the film reacts with protons from an electrolyte it changes color. The corresponding electrochemical reaction is: EQU WO.sub.3 +xM.sup.+ +xe.sup.- .revreaction.M.sub.x WO.sub.3
where M =H, Li, K or Na (faint yellow to colorless) (blue, tungsten bronze).
This electrochromic reaction is sometimes characterized as a change from a bleached, white or colorless state to a colored state. The bleached state has relatively high transmittance and the colored state, relatively low transmittance.
Tungsten oxide, WO.sub.3, has been reported to possess ferroelectric properties. Ferroelectric materials have potential for use in nonvolatile memory devices, that is, devices in which data is retained even when power is cut off. In addition, tungsten oxide may potentially be used for infrared temperature sensors.
Several methods are used to form tungsten oxide films. The methods include sputtering, chemical vapor deposition, and plasma enhanced chemical vapor deposition, in which sub-atmospheric pressure must be maintained. In these methods, large, complex and expensive equipment is needed, and the methods involve significant energy consumption and relatively high operating costs. The films produced by current methods are tungsten oxide films of WO.sub.3, without suboxides, and which are colorless or faint yellow, single color films.
It has been suggested that tungsten containing films be formed by a metallo-organic deposition (MOD) method, wherein an organic metal compound is applied to a substrate which is heated to form the desired metal oxide film. Most MOD processes for deposition of transition metal oxides use carboxylate salts. However, tungsten carboxylates have not been easily synthesized.
It is an object of this invention to provide an MOD method for producing a tungsten oxide film by pyrolyzing at least a portion of an alkyl amine tungstate compound; in which the alkyl amine tungstate compound is a soluble alkyl ammonium salt of tungstic acid; in which time and temperature of heating are varied so as to provide a desired color gradient in the tungsten oxide film and suboxides of the tungsten oxide (WO.sub.3); and which utilizes a precursor which produces a high tungsten oxide yield thereby forming a relatively dense tungsten oxide film.
It is a further object to provide tungsten oxide films which include suboxides of tungsten oxide (WO.sub.3); which have an average ratio of oxygen atoms to tungsten atoms equal to or less than 3:1; which are denser than films produced from currently known MOD precursor compounds; which have a color gradient, that is, a single continuous film marked by regions of different colors; and wherein the regions of color are electrochromic.
In this method, tungsten oxide films are formed without vacuum equipment. Uniform faint yellow to white (colorless) films having essentially no organic matter or impurities are produced, or films having regions of color across the substrate are produced ranging from nearly colorless or faint yellow to deep brown, which offer the potential for inexpensively and controllably darkening windows.